The polyvinyl acetal resin is a generic term used to refer to resins obtained by acetalization of polyvinyl alcohol with aldehyde. The polyvinyl acetal resin is not only favorable in electrical insulating property and excellent in adhesiveness and chemical resistance, but also high in mechanical strength such as flexibility and abrasion resistance, and is therefore used as a coating material or an adhesive in a wide variety of applications. For example, the polyvinyl acetal resin is used as a varnish for enamel wire, a binder for electromagnetic tape, and an adhesive for converging glass fibers and carbon fibers, and furthermore is widely used as a coating material or an adhesive in buildings, aircrafts, and the like constructions. In the case of using the polyvinyl acetal resin in the applications stated above, it is desired that the polyvinyl acetal resin be dissolved in a solvent, thus making a varnish. However, the polyvinyl acetal resin cannot be dissolved in a sufficiently even manner in a generally-used industrial solvent represented by esters such as methyl acetate, ethyl acetate, and butyl acetate, and ketones such as methyl ethyl ketone, acetone, and cyclohexane.
Accordingly, in making the varnish from the polyvinyl acetal resin, there are used, for example, glacial acetic acid, monochloroacetic acid, benzyl alcohol, cresol, xylenol, furfural, dioxane, tetrahydrofuran, pyridine, dichloroethane, chloroform, N-methylpyrrolidone, dimethylsulfoxide, and a mixture solvent of toluene and ethanol. All of these solvents have problems of being high in stimulus property, being relatively high in toxicity to human bodies, being high in environment-polluting property, giving offensive odor, being high in inflammability, etc.
Consequently, in making the varnish from the polyvinyl acetal resin, it is necessary to provide a lot of safety measures by setting up facilities which have satisfactory functions of exhausting air, eliminating harms, eliminating toxicity, removing static electricity, etc., so that the solvent is not brought into contact with a human body or that the solvent is not released to the environment.
Further, the polyvinyl acetal resin is used to obtain film-like electrolyte (gel-typed polyelectrolyte) by gelation of an electrolyte solution in order to achieve an increase in charging/discharging capacity, a decrease in thickness, an enhancement in freedom degree of shapes, and the like purposes in an electrochemical device such as a battery, a capacitor, a solar battery using electrolyte which is called Graetzel cell, and the like device (refer to Japanese Unexamined Patent Publication JP-A 57-143355 (1982), for example). In the related art, when aiming to obtain the film-like electrolyte by use of the polyvinyl acetal resin, a content of the resin needs to be 10% by weight or more based on a total amount of the electrolyte solution, but when molecules of the polyvinyl acetal resin are dispersed into the electrolyte solution in such high concentration, the molecules will disturb the movement of ions. Ion conductivity is thus decreased to a level lower than that of the electrolyte solution itself, resulting in deterioration of electrical load characteristics. A reduced content of the polyvinyl acetal resin can increase the ion conductivity, but leads a decrease in gel strength, which results in loss of such an advantage inherent to the gel-typed polyelectrolyte that a freedom degree of shape of the electrochemical device is enhanced.
Furthermore, it is known that the polyvinyl acetal resin is used as solid-typed polyelectrolyte without impregnating a solvent (refer to Japanese Unexamined Patent Publication JP-A 10-50141 (1998)). However, this electrolyte is also very low in ion conductivity, resulting in a significant decrease in the electrical load characteristics.